One of the most important parts of learning and implementing a programming language for your projects is the official documentation for the language, framework, library or API that you're using or trying to learn; they provide what you need to get to know what they have to offer in detail, with examples, guides, tips, style guidelines, good practices, API reference and other information that the developers and creators of said language or framework wanted you to read.
They are scattered around the web and sometimes we would hope for them to be together in one place that we can access offline, perform a quick search inside them for the topic that we're looking for or just give them a good read when we need. Even the best programmer tends to forget about pieces of syntax and parts of a language, there's no shame in taking a look at the documentation if you keep need a refresher.
This software, developed by Kapeli, offers you plenty of features including snippets, reference for over 150 APIs and languages, the ability to create your own docsets and fuzzy search which is huge to find the topic that you're looking for in no time. Among the languages you can find from popular to not-so-used, if you're a Ruby, Java, C++, NodeJS or .NET programmer, you'll find every piece of documentation here, once you download it you won't have to worry if you're online or not. Most importantly, it has Editor and IDE plugins to find documentation right there in the commodity of your editing tool. The app costs 24.99 USD and you can obtain it here.
The main difference between Zeal, Dash and the last tool is that Zeal downloads the Documentation sets from the official sites, formatted as they come, as if they were scrapped directly from the original sites so, you won't find uniformity and some docsets might take longer to download than others; to download them once you install the software is go to File _ Options _ Docsets, from here, check the ones you want to download and let the process begin; it may take a long time depending on how many and the amount of images they have. The good thing is that it's free and compatible with Linux distros.
If you're not interested in installing software, you can check out Devdocs, they have plenty of APIs and languages and also an offline mode but you need to install the docsets first, a process that doesn't take that much like Zeal does because it's pure text most of the times. Give it a chance and you won't regret it.